The present invention relates to a central differential operation restriction device control device and to a method of operation thereof, for a vehicle adapted for four wheel drive operation and incorporating a four wheel drive power transmission system, and more particularly relates to such a central differential operation restriction device control device and method of operation thereof for a vehicle such as an automobile incorporating such a four wheel drive power transmission system, said central differential operation restriction device control device and method providing control according to the current values of vehicle road speed and of engine load.
The present invention has been described in Japanese Patent Applications Ser. Nos. Showa 61-118234 Showa 61-130958 and Showa 61-200289 (1986), all of them having been filed by an applicant the same as the entity assigned or owed duty of assignment of the present patent application; and the present application hereby incorporates into itself by reference the text of said Japanese Patent Applications and the claims and the drawings thereof; copies are appended to the present application.
Nowadays a greatly increasing number of automotive vehicles are being constructed with four wheel drive transmission systems, because such four wheel drive operation, in which all four wheels of the vehicle are powered from its engine via its tranmsission, is very suitable for driving on poor or slippery road surfaces such as in mud or over bad ground, or upon raods covered with mud, snow, ice, or rain. In other words, four wheel drive operation provides a much higher degree of stability and drivability for the vehicle in conditions where the coefficient of friction between the wheels and the surface upon which the vehicle is riding is relatively low. Also, four wheel drive operation is beneficial for aiding with hill climbing characteristics and high speed stability characteristics. Therefore, the so called full time four wheel drive type of transmission, which remains always engaged to four wheel drive without any episodes of two wheel driving, is becoming more and more popular.
In such a four wheel drive transmission system for an automotive vehicle, it is usual to provide a center differential device for distributing rotational power between the front wheels of the vehicle and the rear wheels of the vehicle, as well as the per se conventional rear differential device that provides differential action between the two rear vehicle wheels and the also per se conventional front differential device that provides differential action between the two front vehicle wheels. Such a central or front - rear differential device is provided in order to provide a differential action between said front vehicle wheels (considered as a pair) and said rear vehicle wheels (also considered as a pair) when the vehicle is turning around a curve, in order to eliminate the possibility of the occurrence of the so called tight corner braking phenomenon created by the difference in the turning radiuses of the front wheels of the vehicle and the rear wheels thereof, when the vehicle is turning around a curve or corner, and also for various other reasons. And such provision of such a central or front - rear differential device is effective for achieving this result. Further, it has been practiced to provide an automatic transmission system to a vehicle which is equipped with such a four wheel drive type transmission. Such a type of structure is disclosed, for example, in Japanese Patent Application Laying Open Publication Ser. No. 56-138020 (1981). Further, it is per se conventional to provide, to such a center differential device, a torque distribution control clutch such as a central differential control clutch, which serves for regulating the distribution of the drive torque produced by the engine of the vehicle between the rear wheels of the vehicle (taken as a combination) and the front wheels of the vehicle (taken as a combination). Such a type of construction is disclosed, for example, in Japanese Patent Application Laying Open Publication Ser. No. 50-147027 (1975) and in Japanese Patent Application Laying Open Publication Ser. No. 55-72420 (1980). And such a torque distribution control clutch such as a central differential control clutch is typically controlled by a hydraulically operated servo device, so that the engagement pressure of said torque distribution control clutch, i.e. the maximum amount of torque that said torque distribution control clutch can transmit, which defines the amount of torque redistribution which said torque distribution control clutch can provide between the rear wheels of the vehicle (taken as a combination) and the front wheels of the vehicle (taken as a combination), is regulated by the magnitude of an actuating hydraulic fluid pressure. And such an actuating hydraulic fluid pressure is typically provided by a control system such as a hybrid electrical/hydraulic control system which may include a microcomputer.
Such a central differential device torque distribution control clutch or central differential device action restriction means is typically provided for the following reason. If even one of the vehicle wheels slips, which may well occur especially when the vehicle is being operated upon a bad road surface such as when it is raining, it is snowing, or when the road is muddy, drive power will be lost, whereupon there is the problem that because of the differential effect of the center differential device the drive power of all wheels will be reduced, and the so called trailblazability of the vehicle will be severly deteriorated. In, therefore, a four wheel drive device having a center differential device, such a central differential restriction device is typically provided.
Now, there is an inevitable difference between the front wheel and rear wheel revolution rates for the vehicle, and this front to rear wheels rotational speed difference is determined by, among other factors, the effective radiuses of said front anddn said rear vehicle wheels, as well as by the turning radiuses of said front and said rear vehicle wheels during an episode of turning around a curve or corner. Further, said effective radiuses of said front and said rear vehicle wheels are determined, among other factors, by the inflation pressures of the tires on said front and rear vehicle wheels, by the wear on said tires, and by the current distribution of the load which is being carried by the vehicle, so that in fact a difference between the front wheel and rear wheel revolution rates for the vehicle can occur not only when the vehicle is turning around a curve or corner but also when the vehicle is going along in a relatively straight line. Further, the higher is the vehicle speed, the greater becomes such a difference between the front wheel and rear wheel revolution rates for the vehicle. Therefore, if during relatively high speed vehicle driving the directly coupled four wheel drive state as described above is provided by engaging the central differential action restriction means such as the central differential clutch, thus providing direct coupling together of the front and the rear vehicle wheels, then the difference in the rate of rotation between front and rear wheels, not being absorbed by the differential action of the central differential device, is likely to cause the occurrence of a circulation torque, the so-called screw up torque, and such a screw up torque is typically accompanied by power losses and vibration. Further such screw up torque is necessarily accomodated by slippage of the vehicle tires upon the road surface, which not only deteriorates the gripping effect of the vehicle wheels upon the road surface but also increases tire wear and deteriorates vehicle fuel consumption.
Now, a concept for controlling the operation of such a central differential device operation restriction clutch has been outlined in Japanese Patent Application Ser. No. 60-280662 (1985), which it is not intended hereby to admit as prior art to the present patent application except to the extent in any case required by applicable law, as follows: the maximum torque transmission capacity of said central differential device operation restriction clutch is increased (so as to provide more restriction of the operation of said central differential device) according to the torque on the input shaft to said central differential device, as well as according to the engaged speed stage of a gear transmission mechanism provided before said central differential device in the power train of the vehicle. However, this operational concept does not necessarily cover all cases of operation: for example, a problem arises when the vehicle is accelerating: when the accelerator pedal of the vehicle is depressed and the engine power output is high, the central differential device becomes locked up according to the operation of its said clutch, and the drive torque is distributed between the front vehicle wheels and the rear vehicle wheels according to the dynamic load distribution, so that the trailblazability and the steerability of the vehicle are improved; but, in high speed vehicle operation with the engine load relatively large, since the operation of the central differential device is still subject to restriction, when there is a difference in the effective radiuses of the front vehicle wheels and the rear vehicle wheels due to loading irregularities of the vehicle or variant tire inflation or wear or the like, then as described above so-called screw up torque will necessarily occur, causing power losses and vibration, slippage of the vehicle tires upon the road surface, thus deteriorating the gripping effect of the vehicle wheels upon the road surface and also increasing tire wear and deteriorating vehicle fuel consumption.